This invention relates to a titanium-clad steel with a high bonding strength and to a method for its manufacture.
Titanium-clad steel has been manufactured by the explosive cladding method, which is advantageous in that the formation of intermetallic compounds is markedly suppressed since the bonding area is not heated to high temperatures, bonding being achieved by heavy working. However, the explosive cladding method has drawbacks regarding noise, productivity, and manufacturing costs.
As a method to eliminate these drawbacks, a rolling method for manufacturing a titanium-clad steel has been developed. In carrying out the rolling method, important factors are (i) an insert to be placed between the steel base metal and Ti cladding, (ii) conditions for assembling with welding as well as for evacuating the assembly, and (iii) heating temperatures for rolling.
Among these factors, it is most important to select an insert particularly suitable for avoiding the formation of brittle intermetallic compounds in the bonding area.
Titanium easily forms intermetallic compounds with various other elements. There are only a few elements such as Mo, Nb, and V that do not form any intermetallic compounds with titanium. However, it is rather difficult to use such materials as an insert, since they are expensive and usually not available in the form of a thin plate. In addition, if they are used as an insert, they are easily broken during rolling.
The purpose of using an insert for manufacturing titanium-cladding steel is to pfevent the formation of intermetallic compounds as well as a brittle titanium carbide layer caused by diffusion of carbon from the base metal steel to the Ti cladding in the bonding area between the titanium or titanium alloy and the base metal steel.
In the prior art, in order to prevent the formation of titanium carbides in the bonding area it has been proposed to use a nickel insert (Japanese Patent Application No. 146763/1985), and a pure iron or ultra-low carbon steel insert (Japanese Patent Application Laid-Open Specification No. 122681/1981), and to carry out decarburization of the base metal steel on the titanium-facing side prior to cladding (Japanese Patent Application Laid-Open Specification No. 220292/1984).
The diffusion rate of nickel in titanium is large and the transformation of Ti from alpha-phase to beta-phase due to the diffusion of Ni into Ti during heating and rolling, further accelerates the diffusion of nickel into the Ti cladding. Therefore, the temperature range which can be employed is very restricted. Thus, a nickel insert is less desirable than ferrous inserts. However, the formation of intermetallic compounds is unavoidable even if a ferrous insert is used. Once formed, such intermetallic compounds grow during spinning, welding, strain-releasing annealing, and other processes which follow cladding, resulting in a deterioration in bonding properties.
Thus, in order to achieve stable and improved bonding properties, it is necessary to develop an insert with which there is no formation of intermetallic compounds in the boundary area between the Ti cladding and the insert, or which can delay the formation and growth of such intermetallic compounds.